1. Technical Field
The present invention relates to various powder-fluidizing and feeding devices for use with coating and spray forming nozzles and guns. The invention discloses new techniques for feeding ultra-fine and nanoscale particles, which are difficult to feed uniformly with the prior art of conventional powder feeders.
2. Background Art
The powder feeder disclosed in U.S. Pat. No. 3,618,828 issued to Schinella uses a vibrating structure to move powder from a receiving surface along a feeding surface to a discharge channel. The primary benefit of this type of powder feeder over prior art is uniform feeding of the powder feedstock without inducing pulsation caused by turbulence in the carrier gas flow. In addition, this type of feeder permits metering of the powder independent of the carrier gas flow rate and properties. The patent further describes the use of a hopper with an outlet channel and a hemispherical cup for metering powder (under the influence of gravity) onto the feeding surface through a smaller port than the outlet channel. The vibratory drive imparts rotary motion to the feeding surface for moving the powder in an outward spiral path along the feeding surface from the receiving surface to the discharge channel. The spacing between the port of the hemispherical cup and the receiving surface is less than the flow control dimension of the port. The feeder structure and hopper of U.S. Pat. No. 3,618,828 is disposed in a chamber for entraining the powder in a carrier gas fed through the discharge channel.
The primary limitation of the powder feeder disclosed in U.S. Pat. No. 3,618,828 is that uniformity of powder metering is highly dependent on the particle size and agglomeration characteristics of the powder. This is particularly true for ultra-fine and nanoscale powders with particle diameters of less than 10 micrometers. For highly agglomerating powders, the hemispherical cup becomes plugged preventing feed from the hopper to the receiving surface. For smooth flowing powders, there is a tendency under the influence of gravity to dump large quantities of powder in an uncontrolled manner onto the receiving surface. Particularly, once powder flow is initiated, uncontrollable feed frequently occurs through all ports and openings in the hemispherical cup resulting in an overflow condition onto the receiving surface.
Feeding of nanometer size particles is considerably more difficult because of the agglomerating aspects ascribed to Van der Waals forces (Handbook of Physics and Chemistry, 68 edition, CRC Press, E-67) acting between the particles. Prior art for dispensing and dispersing nanometer size particles has primarily been limited to colloidal suspensions.
Conventional powder feeding units such as that disclosed in U.S. Pat. No. 4,808,042 to Muehlberger, et al., U.S. Pat. No. 4,740,112 to Muehlberger, et al., U.S. Pat. No. 4,726,715 to Steen et al., U.S. Pat. No. 4,4,561,808 to Spaulding, et al. or in U.S. Pat. No. 4,3,565,296 to Brush, et al., all have difficulty uniformly feeding ultra fine powder. These feeders tend to induce pulsation at low feed rates due to agglomeration or are not able to inject powders into high-pressure guns or nozzles.
The present invention relates to various powder-fluidizing and feeding devices for use with coating and spray forming nozzles and guns. The invention discloses new techniques for feeding ultra-fine and nanoscale particles, which are difficult to feed uniformly with the prior art of conventional powder feeders. The present invention allows powders to be fed into conventional coating and spray forming nozzles and guns, but more importantly into choked supersonic nozzles such as those disclosed in U.S. Pat. No. 5,795,626 issued to Gabel and Tapphorn, U.S. Pat. No. 6,074,135 issued to Tapphorn and Gabel, and friction compensated sonic nozzles disclosed in U.S. patent application Ser. No. 10/116,812 filed by Tapphorn and Gabel on Apr. 5, 2002. These choked nozzles require a high nozzle-inlet pressure which precludes uniform injection of powders using conventional powder feeders. The attribute of the invented powder-fluidizing device that permits injection into high inlet pressure nozzles is powder feeding that is independent of the gas mass flow characteristics. Thus, the powder fluidizing gas can be maintained at a sufficient pressure and flow rate to inject into a nearly isostatic nozzle inlet pressure, while powder is independently metered and entrained into the powder fluidizing gas.
Improvements to the powder-feeding concept, disclosed in U.S. Pat. No. 3,618,828, include sieve plates mounted within a hopper for precise metering of powder into a vibrating bowl. Powder is metered through the sieve plates by a hopper vibrator that is controlled by a level sensor mounted in the vibrating bowl. Other means for metering the powder through conventional pinch, iris, and cone valves are included as a means of metering powders from the hopper into the vibrating bowl. A funnel tube at the base of the hopper extends down into the vibrating bowl to direct the powder agitated through the sieve plates into the vibrating bowl. The funnel tube restricts powder fuming to a small confined volume within the funnel tube as the powder drops to the vibrating bowl surface. This technique eliminates any coupling between the vibrating bowl and the base structure that may dampen or perturb the vibration intensity during operation. Other improvements to the prior art include a means for heating and vibrating powders in the hopper to dissipate agglomeration and clumping of the powder, and methods for improving the precision and accuracy of metering powders from a vibrating bowl through a spiral-ramp groove and feedback control derived from mass loss or particle feed rate measurements.
This invention also relates to several embodiments of portable powder deposition devices for deposition and consolidation of powder particles using friction compensated sonic nozzles such as those disclosed in the aforementioned U.S. patent application Ser. No. 10/116,812 filed by Tapphorn and Gabel on Apr. 5, 2002 or supersonic nozzles as disclosed in U.S. Pat. No. 5,795,626 issued to Gabel and Tapphorn, and U.S. Pat. No. 6,074,135 issued to Tapphorn and Gabel.